Hello all
I posted earlier (#472) measurement of some Fairchild BC337s.

You will find below measurements of BC337s manufactured by ON Semi, purchased from Digikey Canada in early December. Part number BC337-040GOS-ND, Date code April 2010, made in China. Again the circuit used is as shown in Post #72.

The attached picture has your numbers in it, it is fed from the voltage you gave, it generates the output voltage you measured. Then I added your measured voltages (test points, Vbe’s and Vak’s of the LED’s) and I sat back and watched

And my conclusion is, I DO NOT UNDERSTAND IT

There are no Si based transistors with a Vbe of 0.2V (as calculated) or 347mV (as you measured).

The PSU is operating as expected, outputting +18.23V and -18.21V, the current sources seem to work. Now we need some smart person to figure it out.

All soldered up just wondering about how to wire up the loading resistor? Am i right in thinking it just goes from signal to ground. If so can I place it at the input on the pcb

Looking good!! Yes, the input resistors can go on the second pair of input contacts, thats why they are there. They should be connected "floating" from that point, that means, do not connect the input resistor to GND at the resistors but only here.

The attached picture has your numbers in it, it is fed from the voltage you gave, it generates the output voltage you measured. Then I added your measured voltages (test points, Vbe’s and Vak’s of the LED’s) and I sat back and watched

And my conclusion is, I DO NOT UNDERSTAND IT

There are no Si based transistors with a Vbe of 0.2V (as calculated) or 347mV (as you measured).

The PSU is operating as expected, outputting +18.23V and -18.21V, the current sources seem to work. Now we need some smart person to figure it out.

Thats a good one... just spent 30min looking at the same picture, in wonder....

BTW, VBE of Q104 is -1.79V, not 1.79V. It should be fully off. Still, Q103 seems to be conducting, else there would be no output current. But it is doing something between open and short circuit, else V(TP103) would equal the input voltage.

@beetle:

Does this problem happen when you turn on the prereg voltage very quickly (e.g. plugging in a plug) or also when you ramp it slowly (with the voltage button of a lab supply)?

And, are you shure you have no solder joints on the bottom side where they are not supposed to be?

And, can you please check with a resistance meter between the collector tabs of all power transistors (when power is turned off, of course) to see if there is no short through the heatsink?

Power transistors are isolated. I had to to this as I mounted them to the sink with screws. The mosfets are in a plastic case so not needed here. But to make sure, I measured 6MOhm from GND to metalcase.

My soldering looks good. But to make extra sure, I'll wash the PCB in alcohol tomorrow.